The invention relates to a bipolar transistor comprising
a collector region with a first doping type,
a base region with a second doping type,
and an emitter region with the first doping type,
a junction being situated between the emitter region and the base region, and, viewed from said junction, a depletion region extending in the emitter region,
and, said emitter region comprising a layer of a first semiconductor material and a layer of a second semiconductor material.
The invention also relates to a method of manufacturing a bipolar transistor comprising a collector region with a first doping type and a base region with a second doping type, on which an emitter region with the first doping type is formed, said emitter region including a layer of a first semiconductor material and a layer of a second semiconductor material.
U.S. Pat. No. 5,535,912 discloses a bipolar transistor that can suitably operate at high frequencies. Said bipolar transistor has a cutoff frequency of typically 100 GHz, as a result of which the transistor can suitably be used as a component in optical communications networks for transporting 40 Gb/s.
The bipolar transistor is made from silicon and includes a base region with a GexSi1xe2x88x92x strained layer. As the bandgap of GexSi1xe2x88x92x is smaller than that of Si, with the conduction band coinciding with that of silicon, and the valence band energetically moved by xcex94Ev with respect to the valence band of Si, the charge storage in the base region and the emitter region is reduced relative to silicon bipolar transistors at comparable current levels. In order to maximize the speed of the transistor, the percentage of Ge in the base region is as high as possible.
In the known bipolar transistor, the charge storage in the emitter is also reduced, which can be attributed to the fact that the bandgap, viewed from the junction, decreases linearly in the direction of the emitter contact. During operation of the bipolar transistor, minority charge carriers are injected into the emitter region from the base region and accelerated by the internal electric field in the emitter, as a result of which the average residence time decreases.
The GexSi1xe2x88x92x strained layer in the base region causes a change of the bandgap xcex94Ev, as a result of which the collector current increases exponentially by xcex94Ev. As a result, the current gain, which is defined as the quotient of the collector current and the base current, increases substantially. A drawback of a base region with GexSi1xe2x88x92x resides in that the current gain is too high, as a result of which collector-emitter breakdown occurs rapidly. The device is not robust because the bipolar transistor amplifies the current internally. For practical applications, a current gain of only approximately 100 is desired.
In the known heterojunction bipolar transistor, the collector current is reduced by increasing the base doping. In addition, the emitter contact is made of a metal instead of the customarily used polysilicon. The recombination of minority charge carriers at a metal contact exceeds that at a polysilicon contact by approximately one order or magnitude, as a result of which the base current is increased by approximately one order of magnitude.
A drawback of the known bipolar transistor resides in that setting the value of the base current is difficult. As the metal contact borders on the emitter region, and reacts at the interface with the second semiconductor material of the emitter region, the width of the emitter region, viewed from the junction, is highly subject to variations.
As the width of the emitter region of a bipolar transistor intended for high-speed applications is very small, the decrease of the emitter width due to the interface reaction, causing a part of the emitter region to be consumed, is comparatively large. The base current depends very substantially on the width of the emitter region and the interface between the emitter region and the metal. A metal contact leads to a substantial variation in base current between bipolar transistors and hence to a substantial variation in current gain.
It is an object of the invention to provide a bipolar transistor of the type described in the opening paragraph, which enables the current amplification to be very accurately adjustable via the base current.
As regards the bipolar transistor in accordance with the invention, this object is achieved in that the intrinsic carrier concentration of the second semiconductor material exceeds the intrinsic carrier concentration of the first semiconductor material, the layer of the second semiconductor material is situated outside the depletion region, and the second semiconductor material is doped such that Auger recombination occurs.
When the bipolar transistor is in operation, minority charge carriers injected from the base region into the emitter region diffuse from the depletion region in the direction of an emitter contact that borders on the emitter region. In the layer of the second semiconductor material, the intrinsic concentration n, of minority charge carriers is greater than the intrinsic concentration in the first semiconductor material due to a smaller bandgap of the second semiconductor material. In a semiconductor, ni2=np, where n is the concentration of electrons and p is the concentration of holes, so that an increased concentration of minority charge carriers is present in the layer of the second semiconductor material. The physical effect causing an increase in base current is referred to as Auger recombination.
Auger recombination occurs if excess charge carriers recombine in semiconductor material having a high doping concentration. The probability of direct recombination between holes and electrons must not be negligible relative to the recombination speed due to traps (Schottky Read Hall recombination). In the case of Auger recombination, there are three charge carriers that interact with each other, i.e. either two electrons and one hole, or two holes and one electron. Two charge carriers recombine and the third charge carrier takes over the impulse from the incident charge carriers and the energy released by said recombination.
For an n-type emitter, the Auger recombination depends quadratically on the electron concentration and linearly on the hole concentration. Auger recombination contributes dominantly to the base current if the hole concentration is increased by a number of orders of magnitude by the use of the second semiconductor material having a smaller bandgap and hence a higher intrinsic concentration. The increase of the minority charge carriers depends exponentially on the decrease in bandgap. Thus, by accurately setting the bandgap as a function of the composition of the second semiconductor material, the base current can be very accurately set, so that also the current amplification can be very accurately set.
The first semiconductor material in the emitter region may be, for example, InAlAs, and the second semiconductor material may be, for example, InGaAs. An N-type doping for these materials is, for example, silicon, and a p-type doping is, for example, beryllium. Alternatively, silicon may be used for the bipolar transistor comprising Si as the first semiconductor material and a GexSi1xe2x88x92x composition as the second semiconductor material. For the N-type doping use can be made, for example, of As or P, and for the p-type doping use can be made, for example, of B.
Owing to the comparatively high intrinsic concentration, Ge can particularly suitably be used as the second semiconductor material.
Advantageously, the second semiconductor material has a composition that is at least substantially constant over at least a part of the layer. As a result, the bandgap is at least substantially constant over said part as well as the intrinsic carrier concentration. In comparison with a situation where the composition of the second semiconductor material varies, a better setting of the Auger recombination can be achieved in the part of the layer having the at least substantially constant composition, so that the base current that is dominated by Auger recombination can be more accurately set.
Preferably, the first semiconductor material of the emitter region is silicon, and the second semiconductor material is a composition of Si and Ge.
A great advantage of GexSi1xe2x88x92x resides in that, in terms of energy, the conduction band is at the same level as the conduction band of silicon. By virtue thereof, it is possible not to influence the collector current while the base current can be accurately adjusted by means of the percentage of Ge in the second semiconductor layer. As a result of the smaller bandgap of the part of the layer with the GexSi1xe2x88x92x composition, the hole concentration increases in the semiconductor layer. Said increase of the hole concentration depends exponentially on the decrease of the bandgap. The bandgap of GexSi1xe2x88x92x depends substantially linearly on the percentage of Ge. Auger recombination contributes dominantly to the base current if the hole concentration is increased by a number of orders of magnitude by using GexSi1xe2x88x92x.
An additional advantage is that the collector current and the speed of the device, characterized by, inter alia, the cutoff frequency fT, remains unchanged. As the current amplification can be reduced, the emitter-collector breakdown voltage BVceo increases and hence the product of fTxc3x97BVceo increases too.
A further advantage resides in that the current amplification is less sensitive to temperature effects. Bipolar transistors carrying much current, such as power transistors, are internally heated by the current, as a result of which the current amplification increases. As a result of the smaller bandgap of GexSi1xe2x88x92x in the emitter region, the GexSi1xe2x88x92x in the emitter region has a negative temperature effect on the current amplification. This negative temperature effect at least partly compensates for the positive temperature effect, as a result of which the current amplification remains more constant as a function of temperature.
For a high-speed bipolar transistor it is very important that the lifetime of the minority charge carriers is short. The lifetime xcfx84 of the minority charge carriers is approximately xcfx84=1/(xcex93N2), where in the case of silicon xcex93=2xc3x9710xe2x88x9231 cm6sxe2x88x921, and N is the doping in the part of the layer of the emitter region. Thus, at a doping concentration of 3xc3x971020 cmxe2x88x923, the lifetime is typically 0.05 ns. Therefore, to obtain a short lifetime, the doping concentration advantageously is as high as possible, preferably above 3xc3x9710xe2x88x9220 cmxe2x88x923, in the part of the layer including the second semiconductor material.
It is advantageous that the part of the layer comprising the second semiconductor material is n-type doped. In general, npn transistors are faster than pnp transistors. The mobility for electrons is a few times higher than the mobility for holes, so that charge transport of electrons is faster. In addition, the solubility of an n-type doping, particularly As, is much higher than that of a p-type doping, such as B, so that comparatively many charge carriers are electrically active.
In addition, n-type doping enables much shallower emitters to be manufactured, so that charge storage in the emitter is comparatively small. In the manufacture of the transistor, the diffusion of n-type doping atoms, such as As and Sb, takes place at a much lower rate than the diffusion of p-type doping atoms, such as B, so that much steeper doping profiles are manufactured and the emitters become shallower.
The maximum percentage of germanium in the composition of the second semiconductor material is connected with the thickness of the layer. As the lattice constant of germanium (5.66 xc3x85) exceeds that of silicon (5.43 xc3x85), compressive stress occurs in the GexSi1xe2x88x92x layer when this layer is epitaxially provided on a silicon lattice. If the stress in the GexSi1xe2x88x92x layer becomes too large, relaxation of the layer takes place. If the percentage of GexSi1xe2x88x92x practically exceeds 30%, the GexSi1xe2x88x92x layer causes the stress to relax, so that the layer is no longer properly epitaxial and lattice errors and defects occur. Thus, in practice, the percentage of Ge remains comparatively low.
It is important that the minority charge carriers cannot tunnel through the part of the layer, but instead Auger recombine in the second semiconductor material. For this reason, the part of the layer has a width of at least several atomic layers, which, dependent upon the material, typically exceeds several nanometers. For a high Ge concentration of 30%, it is desirable, however, due to the stress relaxation, that the layer is not too thick, i.e. the thickness should typically be below 10 nm.
Preferably, the layer with the second semiconductor material at least substantially adjoins the emitter contact. As, in general, the doping is diffused in the emitter region by diffusion, the concentration of doping atoms of the first type is highest at the surface. Here, Auger recombination in the part of the layer of the second semiconductor is very substantial. By virtue of the dominant effect of the Auger recombination, the base current can be perfectly adjusted by varying the Ge concentration. However, if the doping concentration in the emitter region is at least substantially constant, it is advantageous if the layer with the second semiconductor material at least substantially adjoins the depletion region, because, at said location, the concentration of minority charge carriers is highest.
The bipolar transistor may be part of a semiconductor device comprising a semiconductor body of a first semiconductor material. The invention also relates to such a device.
The semiconductor device may be, for example, an integrated circuit of the bipolar transistor and a CMOS circuit (BiCMOS) or a memory. The semiconductor body of the first semiconductor material may be, for example, silicon, and the bipolar transistor may be a GexSi1xe2x88x92x HBT.
Alternatively, the semiconductor body may be InP and the bipolar transistor may be an InAlAs/InGaAs HBT.
Another object of the invention is to provide a method of manufacturing the bipolar transistor of the type described in the opening paragraph, by means of which the value of the base current is accurately defined.
As regards the method, the object of the invention is achieved, in accordance with the invention, in that a first layer of the first semiconductor material is epitaxially provided on the base region, after which a second layer of the second semiconductor material is subsequently epitaxially provided and doped with a first doping type in such a manner that Auger recombination occurs, and the intrinsic carrier concentration of the second semiconductor material exceeds that of the first semiconductor material.
The first semiconductor material with the comparatively larger bandgap and the smaller intrinsic carrier concentration may be, for example, InAlAs, and the second semiconductor material may be InGaAs. The layers are epitaxially grown on the base region, for example, by means of gas source molecular beam epitaxy. For the emitter use can be made, for example, of a heavily doped n-type emitter, the doping being provided, for example, by means of ion implantation and diffusion. The doping concentration at which Auger recombination occurs depends on the semiconductor material. The doping concentration is comparatively high, which generally leads to bandgap narrowing. The second semiconductor material may be, for example, a III-V semiconductor, germanium, or a compound of germanium, such as SiGe.
Advantageously, the composition of the second semiconductor material over the second layer is at least substantially constant. A constant composition of the material has the advantage that the bandgap is at least substantially constant and hence the intrinsic concentration of charge carriers is also at least substantially constant. This enables the magnitude of the Auger recombination to be accurately adjusted.
An advantageous combination of semiconductor materials in the emitter region comprises Si as the first semiconductor material and a composition of Si and Ge as the second semiconductor material. A great advantage resides in that the epitaxy cannot only be carried out using a slow growth method, such as MBE, but also by means of a fast deposition method, such as chemical vapor deposition. During the deposition process, the doping can be provided in situ. In this manner a substantially constant doping level in the second semiconductor material is guaranteed. As the whole emitter region has the same doping type, for example the n-type, it is advantageous to n-type dope the first semiconductor material, so that in the growth process of the second semiconductor material with the same n-type doping, it is not necessary to switch on the gases, as a result of which the doping is more uniform and autodoping does not occur.
The doping level that can be provided in situ in the semiconductor materials depends on the temperature of the deposition and on the doping atom. The solubility of As is related to the temperature at which deposition takes place. P has a lower solubility product and hence is less suitable for high doping levels in the emitter region. Sb has a comparatively low solubility product, however, in the case of clustering, a doping concentration of 1xc3x971020 cmxe2x88x923 can be attained. An advantage of Sb resides in that the diffusion coefficient is comparatively low, so that steep profiles can be obtained.
Alternatively, the high doping level in the emitter region can be provided in that an emitter contact is formed on the emitter region by providing a polysilicon layer having a doping of the first doping type on the emitter region, and the second layer is doped through outdiffusion of the doping atoms of the polysilicon layer. The doping may have been provided in the polysilicon during the deposition process, however, in general the doping is provided in the polysilicon layer through ion implantation. Subsequently, during a step at a high temperature of approximately 900xc2x0 C., the doping atoms are diffused from the polysilicon layer into the emitter region. It is important to make sure that the diffusion time at high temperatures is short so as to obtain a shallow emitter region. To achieve this, use is often made of rapid thermal annealing RTA, or laser annealing. The doping is brought to high temperatures for a few seconds only, as a result of which outdiffusion is small.
In an advantageous method of manufacturing a semiconductor device comprising a semiconductor body of the first semiconductor material, the collector and the base region are generally provided on a substrate. In the case of InAlAs/InGaAs transistors, the semiconductor material is InP.
The bipolar transistor of InAlAs/GaAs may be integrated with InP devices so as to form an optoelectronic circuit that can very suitably be used as a component in an optoelectronic network. A semiconductor device comprising a semiconductor body of Si and a bipolar transistor of silicon can particularly suitably be used for BiCMOS and embedded memories.
The bipolar transistor is manufactured in a CMOS process requiring only a few additional masking steps.
The emitter region can be selectively epitaxially grown in an emitter window of, for example, oxide and/or nitride, as described in WO 9737377.
Growing the emitter in an emitter window by means of selective epitaxy is advantageous because it does not require additional masking steps. As selective growth is difficult, the emitter region may alternatively be formed with a differential epitaxial layer, as described in U.S. Pat. No. 5,821,149.
In general, it is advantageous to form the emitter at a late stage in the process, so that the thermal budget of the emitter is small, as a result of which the emitter remains shallow and the doping atoms are not electrically deactivated.
To form the emitter region in the semiconductor device, the methods described hereinabove apply, and all combinations also apply to the semiconductor device.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.